In cryogenic air separation plants, air is compressed, purified of higher boiling contaminants such as water vapor and carbon dioxide and then cooled to a temperature suitable for the cryogenic distillation of the air. The air is then typically rectified within a double column air separation unit having a higher pressure column to produce a nitrogen-rich vapor column overhead and a crude liquid oxygen column bottoms, also known as kettle liquid. The crude liquid oxygen column bottoms is further refined in a lower pressure column to produce an oxygen-rich liquid column bottoms and another nitrogen-rich column overhead. The oxygen-rich liquid column bottoms of the lower pressure column is used to condense the nitrogen-rich vapor produced in the higher pressure column and commonly, resulting nitrogen-rich liquid is used to reflux both columns. The oxygen-rich liquid column bottoms is partially vaporized as a result of the condensation of nitrogen-rich vapor to provide boilup in the lower pressure column. The products from such a plant can be nitrogen and oxygen vapor and liquid products. Additionally, if an argon product is desired, an argon column can be attached to the lower pressure column to refine an argon product.
As mentioned above, prior to cooling the air to cryogenic temperatures that are suitable for conducting the distillation, the air must be purified of higher boiling contaminants, for example, water vapor and carbon dioxide. Either of these components could freeze during the cooling of the air and accumulate within heat exchange passages of a main heat exchanger used for such purposes. Water vapor and carbon dioxide is therefore removed by adsorption processes and systems that utilize beds of adsorbent operating in an out of phase cycle to adsorb such contaminants. While one adsorbent bed is adsorbing the impurities, another of the beds is regenerated. In air separation plants, adsorption units are provided that operate in accordance with a temperature swing adsorption cycle. In temperature swing adsorption cycles, the adsorbent beds are regenerated with the use of heated gas, typically, waste nitrogen produced by the air separation plant.
An example of a temperature swing adsorption cycle that would be useful in purifying the air in an air separation plant is described in U.S. Pat. No. 5,846,295. In this patent, air is compressed in a main air compressor 6 to a pressure that can be anywhere from 28 to 250 psia. The air is cooled in heat exchangers 8 and 10. The cooling of the air will condense some of the water vapor content of the air. The compressed air is then supplied to an inlet manifold 12 from where it is fed to one of two adsorbent vessels 2 or 4, depending upon which of the vessels is on line and which is being regenerated. Purified air is supplied from an outlet to the cold box of the air separation plant that houses the distillation columns. The adsorbent vessels 2 and 4 contain an alumina adsorbent that will adsorb the water vapor and carbon dioxide. Once an adsorbent vessel is loaded with such impurities, accumulated high pressure gas within the adsorbent bed is allowed to vent in a depressurization or blow down step and dry nitrogen rich waste gas from the cold box is then introduced into a heat exchanger 66 where it is warmed and supplied to the adsorbent bed to be regenerated. The impurities will desorb from the adsorbent due to the heating of the adsorbent by the warm dry nitrogen rich waste gas. Once, the bed has been regenerated, it is repressurized with part of the compressed gas produced by the main air compressor 6 and brought back on-line. The on-line adsorbent bed is then regenerated as described above.
As can be appreciated, in the fabrication and operation of an air separation plant, it is desirable to reduce both fabrication costs which can be capitalized over the life of the plant and ongoing running costs that are incurred through electrical power usage. By operating the temperature swing adsorption unit at conventional pressures such as have been mentioned above, to at least a certain extent, fabrication and operational costs are reduced over operating temperature swing adsorption units at higher pressure. In this regard, one cost arises from the material making up the vessel that houses the adsorbent. Lower operational pressures will allow the adsorbent vessel to have a thinner sidewall than would be the case had the adsorption been conducted at a higher pressure. Thus, to such extent, fabrication costs of the adsorbent beds are reduced when designed to operate at lower pressure. As mentioned above, costs also arise from the ongoing operational costs incurred through electrical power consumption. The depressurization or blow down step that is conducted during adsorbent bed regeneration also represents a cost because the depressurization of the adsorbent bed represents a loss of high pressure air that had a specific power cost related to the compression of the air that is vented. The power costs incurred in compressing the air at a lower pressure are less than the costs involved in compressing the air to a higher pressure. Therefore, by operating the temperature swing adsorption process at a lower pressure, the costs involved in depressurizing the adsorbent bed are less than would otherwise be incurred at a higher pressure.
As will be further discussed, among other advantages, the present invention provides a compression system for an air separation plant having a temperature swing adsorption unit situated within a location of the compression system to allow the adsorption to be conducted at a higher pressure than that contemplated by the prior art, namely, between 400 psia and 600 psia and with a reduction in both fabrication and ongoing operation costs over compression systems in which the temperature swing adsorption unit is operated at lower pressures.